1. Field of the Invention
This invention relates generally to printing heads which include thermal printing heads and LED printing heads for example. More particularly, the present invention relates to a line-type printing head in which connection terminals used for electrical connection to an external control circuit are arranged only locally of the head. The present invention also relates to a printer incorporating such a printing head.
2. Description of the Prior Art
As is well known, a typical line-type thermal printing head comprises an elongate head substrate which is provided with a line of heating dots along a first longitudinal edge (one longitudinal edge) of the substrate, and an array of drive IC's along a second longitudinal edge (the other longitudinal edge) of the substrate for driving the heating dots. The head substrate is also provided with comb-like connection terminals adjacent to the second longitudinal edge of the substrate for receiving power supply and various signals from an external control circuit. A wiring conductor pattern is formed on the substrate for electrically connecting the connection terminals to the respective drive IC's.
The head substrate is usually attached on a rigid support plate which functions also as a heat sink. A connector board (often referred to as "flexible board") is mounted on the support plate to partially overlap the head substrate. The connector board, which is also formed with a wiring conductor pattern, has comb-like connection terminals in corresponding relation to the connection terminals of the head substrate for connection to the external control circuit.
Further, a presser cover substantially equal in length to the head substrate is fixed on the support plate for pressing the connection terminals of the flexible connector board into intimate electrical contact with the connection terminals of the head substrate. Fixation of the presser cover is usually established by screws spaced suitably along the entire length of the support plate.
The conventional line-type thermal head having the above-described structure is known to have the following disadvantages.
First, the provision of the presser cover inevitably increases the thickness of the thermal head. Thus, it is difficult to realize a sufficient size reduction required for recent facsimile machines and printers.
Secondly, a problem of thermal bending occurs in the conventional thermal due to the so-called "bimetal phenomenon", thereby seriously deteriorating the printing quality. The bimetal phenomenon is caused by difference in linear thermal expansion between the support plate and the presser cover because the support plate receives heat immediately from the head substrate while the presser cover receives heat from the support plate indirectly through the screws with a time lag.
U.S. Pat. No. 4,963,886 to Fukuda et al. discloses an improved line-type thermal printing head wherein connection terminals are arranged locally only in a limited central portion of a head substrate which is attached on a support plate, and a flexible connector board is correspondingly reduced in length. Due to such a structure, a presser cover needs only to press the connector board only in the limited central portion of the head substrate, and both ends of the presser cover are made to move longitudinally of the head substrate. Thus, even if the support plate and the presser cover are linearly expanded to different degrees, the thermal head can be prevented from thermally bending to the bimetal phenomenon.
However, the prior art thermal head disclosed in the above U.S. patent still has the following disadvantages.
First, the prior art thermal head retains the basic structure wherein the flexible connector board is pressed against the head substrate by the presser cover. Thus, the provision of the presser cover increases the overall thickness of the thermal head.
Secondly, since the connection terminals of the head substrate are arranged in the limited central portion of the substrate, a common electrode for the heating dot line must have a pair of leg portions extending from both ends of the head substrate to the central portion thereof. Thus, the overall length of the common electrode becomes inevitably large to increase voltage drop along the common electrode, consequently giving adverse influences on the printing quality. The problem of voltage drop is particularly significant because the common electrode must pass a relatively large current.
In the third place, because of the central arrangement, the connection terminals of the head substrate need be located in a strip-like portion between a drive IC array and a longitudinal edge of the head substrate. Further, the leg portions of the common electrode must extend in the same strip-like portion of the head substrate. Thus, the width of the strip-like portion need be sufficiently increased to enable formation of the connection terminals and the common electrode leg portions together with a wiring conductor pattern for connecting the connection terminals to the drive IC array. As a result, the width of the head substrate must be inevitably increased in spite of the requirement for a size reduction.